Despite the rise of digital radio, FM radio receivers are still very popular and are increasingly seen in mobile telephones and other electronic handheld devices such as MP3 players. The introduction of FM radio receivers into such devices has introduced a new set of noise problems which had not previously been a priority for FM designers, particularly with respect to stereo FM broadcasts, which is the dominant format used today.
In order to ensure that stereo broadcasts are compatible with mono receivers having just one speaker, stereo FM is encoded in a manner that allows a mono receiver to easily extract a channel which contains a mix of the left and right stereo channels. Stereo FM is encoded using two channels, one being the sum of the left and right stereo channels (L+R) and the other being the difference of the left and right channels (L−R). A mono receiver uses just the L+R channel for playback. A stereo receiver can add the L+R and L−R channels to obtain the left channel and subtract the L−R signal from the L+R signal to obtain the right channel.
As shown in FIG. 1, when suitably demodulated, the main L+R channel occupies a frequency range of 30 Hz to 15 kHz. Sub-channel L−R is a double-sideband suppressed carrier (DSBSC) signal using the baseband range of 23 kHz to 53 kHz. A pilot tone is also present at 19 kHz and is used to allow the sub carrier to be demodulated with the correct phase. At the FM transmitter, a composite signal of the main channel, sub-channel and pilot tone can be used to modulate the carrier.
FIG. 2 shows a typical FM receiver arrangement. The FM signal is received from the FM transmitter via the antenna, amplified using a low noise amplifier (LNA) and then mixed with a local oscillator (LO) signal. The LO signal is generated using a phase locked loop (PLL). The LO signal is used to down convert the received signal. The down-conversion processes the received signal to create an intermediate frequency (IF) signal where the frequency spectrum of the wanted signal component is located at frequencies that are convenient for further processing. The IF signal is then filtered to remove unwanted noise, before it is demodulated by an FM demodulator. The resulting signal is then filtered to produce a separate L+R channel. The L−R channel is generated by translating the stereo sub-carrier to base-band, whereby its spectrum is ‘shifted’ in frequency by 38 kHz. This shift is performed by mixing the stereo sub-carrier with a 38 kHz signal using a mixing circuit.
The individual left and right channels are then generated by summing the L+R and L−R channels as described above. E.g. The L+R and L−R channels are summed to obtain the left channel and the L−R signal is subtracted from the L+R signal to obtain the right channel.
One problem with this system is a type of noise resulting from PLL ‘reference spurs’. The signal generated by a PLL usually has a number of spurious noise levels. The specification of a RF system making use of a PLL usually takes this into account. These spurious noise levels may occur for several different reasons. A ‘reference spur’ is so named as it results from feedthrough from the reference signal used by the PLL. The spurs are caused by imperfections in the PLL components, such as a mismatched propagation delay in the phase frequency detector, mismatches in the charge injection and current, and leakage current in the VCO tuning node. Consequently, as the PLL is unable to generate an output signal perfectly in phase with the reference signal, an oscillation of the phase of the generated signal occurs as the VCO continually corrects to bring the generated back or forward to match the reference signal. This oscillation has a frequency matching the reference frequency. Therefore, the resulting signal generated by the PLL has, not only an AM modulated component, but an FM modulated component.
Therefore, the reference spur is a sinusoidal phase modulation of the local oscillator signal at the frequency of F_ref (the reference frequency used for the PLL clock). In the FM receiver described above, when the received FM signal is mixed with the LO signal featuring the reference spur, a sinusoidal phase modulated noise signal at a frequency of F_ref is included in the resulting signal. When this signal is FM demodulated, a noise signal at frequency F_ref results. Where F_ref is within the band occupied by either the stereo main channel or sub-channel, this can result in audible noise in the fully demodulated FM stereo signal.
FIG. 3 shows an example of the reference spur noise problem described above. Many mobile phones currently include FM stereo broadcast receivers and supply to them a clock with a frequency of 32768 Hz. This clock frequency is commonly used in mobile phone architectures. Where the FM receiver incorporates a PLL that generates an LO signal for down-conversion, it is often convenient to use the 32768 Hz clock as a reference signal for the PLL. A 32768 Hz clock used as a reference signal for the PLL can create a 32768 Hz LO spur, resulting in a sinusoid of 32768 Hz being added to the stereo multiplex signal recovered in the receiver through FM demodulation. This unwanted 32768 Hz sinusoid lies within the 23 kHz-53 kHz band occupied by the stereo sub-carrier. The signal processing in the receiver includes translating the stereo sub-carrier to base-band, whereby its spectrum is ‘shifted’ in frequency by 38 kHz to produce the L−R signal. Once the FM signal has been fully demodulated, the unwanted sinusoid appears in the L−R signal at a frequency of 38000 Hz−32768 Hz=5232 Hz. The L−R signal is subsequently added to the L+R signal to produce stereo audio consisting of ‘left’ and ‘right’ signals. Hence the unwanted sinusoid is audible in the left and right channels at 5232 Hz.
Previous approaches to addressing this problem have involved refining the design of the voltage controlled oscillator (VCO) in the PLL to minimise the size of the reference spurs and so reduce the spurious audio noise. These techniques can require a great precision in the manufacture of the components and result in a higher per-unit cost.
What is needed is a method of reducing or eliminating the noise resulting from the reference spurs without the use of more expensive components.
According to a first aspect of the present invention there is provided an apparatus to reduce noise in a stereo FM broadcast received via an antenna, the apparatus comprising: a frequency translator configured to translate the received stereo FM broadcast to an intermediate carrier frequency, a demodulation unit configured to demodulate the translated FM signal so as to form left-plus-right and left-minus-right AM signals, a filter configured to form a filtered signal by filtering one of the AM signals, so as to suppress a sub-band of that signal containing unwanted tones, a summing unit for summing the filtered signal and the other of the left-plus-right and left-minus-right signals to produce a stereo audio signal.
According to a second aspect of the invention, there is provided a method of filtering noise from a stereo FM signal, the method comprising the steps of: demodulating an FM stereo broadcast signal into the left-plus-right and left-minus-right AM signals, filtering one of the AM signals, so as to suppress a sub-band of that signal containing unwanted tones, summing the filtered signal and the other of the left-plus-right and left-minus-right signals in such a manner as to produce a stereo audio signal.